Experimental reproduction of the martian weathering profiles argues for a dense Noachian CO<sub>2</sub> atmosphere

On Mars, mineral sequences have been detected and they are composed of a top layer of Al-rich clay minerals, then (Al, Fe)-rich clay minerals and a bottom layer composed of (Mg, Fe)-rich clay minerals. By analogy with Earth, such sequences are interpreted as weathering profiles formed by the interaction of acidic solutions in equilibrium with the atmosphere and the parent rock. Thus, understanding of the aqueous solution composition leading to the above mineral description allows deciphering the atmosphere composition. We designed an experimental column system with three levels containing powdered basaltic rock to test the influence of different acidic fluids on the mineralogical formation. Five solutions were used: H2SO4 and HCl at pH 3 in equilibrium with N2 atmosphere, pure water in equilibrium with 0.1 and 1 atmospheric pressure CO2 leading to pH values of 3.9 and 4.4, respectively and a H2SO4 solution at pH 3 in equilibrium with 0.1 atmospheric pressure CO2 leading to a pH value of 2.98. The results obtained show that the content of Al-rich clay minerals and the evolution from Al, (Al, Fe) to (Fe, Mg)-rich clay minerals formed are better reproduced with an originally high pCO2. Hence, we suggest that acidic alteration driven by a dense CO2 atmosphere reproduced better the observed martian weathering profiles. The experiments involving CO2 led to the formation of carbonates. Their identification by near infrared (NIR) detection methods is challenged, because the laboratory NIR spectra acquired on the experimental products show that: (i) the absorption bands related to carbonates are very weak, and (ii) the strongest feature at 3.95 μm is beyond the CRISM NIR range. Such carbonate formation is consistent with the recent carbonate detection at a planetary scale in weathering profiles, which goes toward that the weathering profiles could have been formed under a dense CO2-rich atmosphere as suggested also by climatic models